Depressed collector operation of electron beam device



March 2, 1965 R. J. VON GUTFELD ETAL DEPRESSED COLLECTOR OPERATION 0F ELECTRON BEAM DEVICE Filed June 17, 1960 '5 sheets-sheet 1 CATHODE IDIVENTORS ROBE/P7' CHYO CHEN WANG ATTORNEY j VU/V GUTFELD Marek 2, 1965 R. J. voN GUTFELD ETAL 3,172,004

l nEPEEssEn' coLLEcToE OPERATION 0E ELEcTEoN BEAM DEVICE FiledJune 17. 1960 :s sheets-sheet 2 FIG. 2.

l K=1 I msc THlcKNEss =1crX I El NORMALIZED AXIAL DISTANCE FROM FIRST POTENTIAL PLANE Raaf/Er II/NVlFoN/ITCRUSTFELD CHAO cH/v WA /vG ATTO NEY FIG.7.

NORMALIZED AXIAL DISTANCE FROM FIRST POTENTIAL PLANE March 2, 1965 R. J. VON GUTFELD ETAL DEPRESSED COLLECTOR OPERATION OF ELECTRONBEAM DEVICE Filed June 17. 1960 3 Sheets-Sheet 5 *MM A Il l "\\r\\ Q N l, fin Ll, l

g1, uw l y I e R 9. `.7 g u- I'illllknl ROBERT NY/ENTFELD CHA 0 CHE N N G ATTORNEY j United States Patent yO 3,172,004 DEPRESSIED CLLECTOR UPERATIN @El ELECTRON BEAM DEVICE Robert I. von Gutfeld, New York, and Chao Chen Wang,

Mineola, NY., assignors to Sperry Rand Corporation,

Great Neck, NX., a corporation of Delaware Filed .lune 17, 1960, Ser. No. 36,986 1o Claims. (Si. 315-3) This invention relates to electron beam devices, and in particular to means for permitting the operation of such a device with a depressed voltage on the beam collector electrode of the device.

ln a linear electron beam device such as a traveling wave tube or a klystron, for example, it has been a customary practice to electrically bias the collector electrode at or very near the potential of the microwave structure of the tube. The KF. properties of the device require that the microwave structure be at a high potential with respect to the cathode of the tube. This in turn requires that the collector also be at a high potential with the result that the electrons in the beam have considerable energy when they strike the collector. This energy largely is converted into heat in the collector and causes considerable power loss and low efficiency of operation of the tube. ln high power tubes additional cooling means must be provided to cool the collector. This cooling equipment is often expensive, bulky, and requires a considerable amount of power to operate. A further disadvantage of operating the collector at a potential near the potential of the microwave structure is that the high velocity electrons striking the collector generate X-rays and give rise to a shielding requirement.

Because of the above-noted problems, considerable effort has been devoted to attempts to operate linear electron beam tubes such as traveling wave tubes and klystrons with a depressed potential on the collector, i.e., the collector potential is considerably lower than the potential of the RF. structure. Although the above-mentioned difficulties are largely solved by lowering the potential on the collector, a new problem is created because the collector potential no longer is high enough for the collector to attract all of the secondary electrons which are produced when the beam strikes the collector. The secondary electrons are drawn to the higher potential RJF. structure and cause noise and instability -in the RF. portion of the tube. Additionally, secondary electron current flow in the RF. section may cause excessive heating in that portion of the tube.

It therefore is evident that the successful operation of a linear electron beam tube with a depressed collector potential requires that means be provided to prevent secondary electrons from leaving the collector region of the tube. Many attempts have been made to suppress sec* ondary electron current ow in a depressed-collector electron beam tube. For example, various types of Faraday cages have been employed, and positive potential electrodes have been positioned intermediate the RF. structure and the collector to collect the secondary electrons on this intermediate positive electrode. In general, known attempts to suppress secondary electrons have required relatively complex structures, and/ or electrical and magnetic means which consume appreciable power.

Therefore, an object or this invention is to provide simple and economical means for suppressing secondary` electron flow in an electron beam device operating with its collector electrode at a depressed potential.

A further object of this invention is to provide a simple structure which consumes substantially no power in suppressing secondary electron current flow in a linear elec tron beam device operating with a depressed collector potential. 1

ICC

Another object of this invention is to provide a simple electrostatic means for suppressing secondary electron current ilow between two electrodes in an electron beam device.

Another object of this invention is to provide a simple structure to establish in the region adjacent the collector electrode of an electron beam device a potential distribution which has a minimum potential suioiently below the collector potential to prevent secondary electrons from leaving the immediate region of the collector.

The present invention will be described in connection with the accompanying drawings wherein:

FlG. l is a diagrammatic illustration of a traveling wave tube constructed according to the present invention;

FG. 2 is a series of curves used to help explain the theory of operation of the present invention;

FlG. 3 is a sectional view of a klystron tube constructed in accordance with the present invention;

FG. 4 is a sectional view of the collector portion of an electron beam tube illustrating another embodiment of the present invention;

FGS. 5 and 6 are sectional views of the collector regions of electron beam tubes illustrating still other embodiments of the present invention; and,

FIGS. 7 and S are graphs from which information may be obtained for constructing an electron beam tube in accordance with the present invention.

Before undertaking a more detailed description of the embodiments of the present invention, it will be helpful first to examine the theoretical background from which the present invention evolved. In an article by Fay, Samuel and Shockley entitled On the Theory of Space Charge Between Parallel Plane Electrodes, appearing on page 49 of the January 1938 issue of The Bell System Technical Journal, the authors present families of curves which represent the different potential distributions which can exist in an evacuated reg-ion between two infinitely extending parallel planes of known potentials when electrons having velocities corresponding to the potentials of the planes are injected perpendicularly into the region through one of the planes. It is assumed that homogeneous space charge conditions exist between the planes and that all the electrons move perpendicularly between the planes, and that the electrostatic potential between the planes is a function only of the distance between the planes. For one set of conditions wherein the potentials of the Vplanes are relatively high and the separation between the planes is relatively short, it is shown that for certain electron beam densities (or perveances) and diameters, and when second plane is at a lower potential than the lirst plane, the potential distribution as a function of distance between the planes is represented by a curve which initially has a negative slope at distances close to the first plane, then falls to a potential minimum at some distance between the planes, said minimum being sufficiently above cathode potential to prevent the formation of a virtual cathode, and finally the curve takes a positive slope as the second plane is approached. With these conditions present, complete electron beam current flow between the planes is achieved.

In designing electron beam tubes such as traveling wave tubes and klystrons to operate with depressed potentials on the collector we employ an extremely simply structure by which we are able to establish a potential distribution of the type just described between the end of the microwave structure of the tube (the first plane) and the collector electrode (the second plane), such that a potential minimum is established close to the collector to act as a suppressing iield to prevent the secondary electrons generated at the collector from passing through said potential minimum, thus preventing them from reaching the microwave section of the tube. These low velocity secondaries are repelled back to the collector of the tube and are collected.

With given electron beam perveance K and diameter 2r and for a selected percentage of collector potential depression o, a particular spacing x between the two planes (microwave structure and collector) must be chosen, and by a simple structure preferably at cathode potential we establish the beam boundary conditions to assimilate a selected one of the potential distributions which is shown in said article to exist between infinitely extending parallel planes having homogeneous space charge therebetween; the specific potential distribution chosen being one which has a potential minimum at least 30 volts below the potential of the second plane and above cathode potential. As set forth in Equations 24 and 25 of said article, the separation x between the two planes may be expressed as and the distance a min. from the first plane to the potential minimum may be expressed as wherein fp is the ratio of the potential at the second plane to the potential at the first plane, a is the ratio of the potential minimum to the potential at the tirst plane, and

As used herein, the symbols x and x0 correspond respectively to the symbols S `and S0 as used in said article.

As is known, in the presence of homogeneous space charge between infinitely extending parallel planes the transverse voltage gradient is zero everywhere between the planes, so that in the practical situation, which assimilates the described theoretical condition, the potential minimum will exist within the beam as well as `along the boundary, thus acting as an effective suppressing field within the beam. The potential minimum is sutiiciently above cathode potential to prevent the formation of a virtual cathode so that substantially complete primary beam current flow is obtained.

In a preferred embodiment of the present invention the field forming means takes the form of a thin apertured disc disposed coaxially about and outside of the beam and electrically biased at cathode potential. The diameter of the disc aperture, and its axial position between the collector and the end of the microwave structure is best Vdetermined experimentally. According to the present invention, the `apertured disc should appear as a line conductor and therefore should not have appreciable axial dimensions.

The present invention is not limited in its application to a linear electron beam tube wherein the two electrodes (microwave structure and collector) are, or assimilate, parallel planes. The invention also may be practiced with concentric cylindrical electrodes or concentric spherical electrodes. ln both of these latter cases electrostatic field forming means are positioned between the electrodes, in a manner to be described, in order that the above-mentioned type of potential distribution is established along the beam boundary between the electrodes.

Referring now more particularly to FIG. 1, a traveling wave tube constructed according to the present invention will be described. Except for the structure to the right of the R.F. output section of the tube, which will be explained in detail below, the tube is of conventional design and is comprised of a metallic cylindrical shell itl having a cathode lll of conventional design at its left end. A voltage source Vg, supplies current to a heater element associated therewith. A helical slow-wave propagating structure l2 is coaxially disposed throughout the central portion of metallic cylinder lll and is coupled at its left end to a coaxial line input section 13, and is coupled at its right end to coaxial line output section 14. Conductive horn members l5 and i6 are coaxially disposed about opposite ends of helix l2 to match the helical slow-wave structure l2 to respective input and output coaxial line sections i3 and 3.4. internal magnetic pole pieces 17 and 13 are respectively disposed adjacent the input and output ends of helix 12., and in cooperation with an external source of magnetic liux, not shown, provide an axial magnetic field to focus the electrons emitted from cathode il into `an electron beam which passes through helix l2.

To the right of magnetic pole piece i8 is the novel arrangement which permits the successful depressed potential operation of collector electrode 19 in accordance with the present invention. ,intermediate pole piece 18 and collector lll, and electrically insulated from each, is an electrostatic field forming means comprised of a thin apertured disc Ztl disposed coaxially about the longitudinal axis of the tube. The axial position and the aperture diameter of thin .apertured disc 20 is best determined experimentally in a manner to be described hereinbelow. Field forming means 20 is electrically insulated from magnetic pole piece l and collector l@ by ceramic rings El and 22. ln accordance with standard electron tube manufacting procedures the ceramic members 2l and 22, apertured disc 2b and collector 19 all may be joined by vacuum-tight seals, or may be surrounded by a vacuumtight envelope.

As examples of operating potentials applied to different portions of the traveling wave tube structure, cathode lll is illustrated as being operated at a potential of 10,000 volts. Magnetic pole pieces 17, i8 and helix l2 are operated at ground potential. Collector i9 is not operated at ground potential as in `the usual practice, but in this example is operated at 6500 volts, representing 65% potential depression. These potentials are merely illustrative and in no way are intended to be limitations on the operating conditions in traveling wave tube constructed and operated in accordance with the present invention. The depressed potential applied to the collector electrode, which is illustrated as being Within 35% of cathode potential, is not to be interpreted as the optimum `potential depression which can be achieved in a device employing the present invention.

in accordance with the present invention the function of thin apertured disc 2Q is to electrically appear as a line source of potential which establishes at the beam boundary in the region between pole piece i8 and collector 19 a potential distribution assimilating the above-mentioned particular potential distribution having a potential minimum in the space charge region between infinitely extending parallel planes. As will be explained, the spacing between pole piece l and collector i9 is chosen so that the potential distribution along the beam boundary will have the desired potential minimum, or potential well, sufficiently below the potential of depressed collector i9 to prevent secondary electrons created at collector 9 from passing through said minimum, and thus causing them to return to collector 19 where they are collected. When the desired condition is established along the beam boundary the potential gradient is zero through the beam in a direction perpendicular to the beam edge. This condition follows from the fact that, so far as the beam is concerned, it behaves as though it were in the hornogeneous space charge region between infinitely extending parallel planes, wherein the transverse potential gradient is zero everywhere between the two planes. Therefore the above-mentioned potential minimum will exist within the beam, thus providing an effective suppressing field for repelling the secondary electrons within the beam.

The type of potential distribution which the field forming means comprised of thin apertured disc 2t? must establish is referred to in the above-mentioned article `as a type C A,.st'entiz-.l distribution. A family of curves representing several potential distributions of this type is illustrated in FIG. 2 wherein the ordinate axis of the curves is in terms 1, wherein is equal to V/ V1, V1 being the potential at the rst plane (pole piece 18) through which the electrons enter into the region, and V is the potential at any position in the beam between the planes. The abscissa axis of the curves is in units of a expressed in terms of x0, wherein J being the current density of the electron beam. Quite often the characteristics of the beam are given in terms of its radius r and its perveance K. Since current density J is related to beam perveance and beam radius by the expression the above expression for x0 may be rewritten as ,.1/2.r Ki/z In electron beam tubes such as traveling wave tubes and klystrons, the beam current density, or perveance, and beam diameter are determined by the RF. properties of the tube, so that in our present consideration of the electron beam collecting problem, the quantities l, K and r already will have been iixed.

Correlating the curves of FIG. 2 to the problem solved by the present invention it is seen from the curves that the potential distribution :along the beam edge must decrease lat increasing distances from pole piece 18, then reach a potential minimum at a potential above cathode potential, and iinally rise to some potential below said pole piece potential at the collector. The most probable energy `distribution of the secondary electrons produced at the collector will be somewhere between thirty and forty volts. Therefore, the potential minimum, or well, should be at least thirty to forty volts below the potential of the collector. The potential minimum must be above cathode potential to `avoid the creation of a virtual cathode which would block the primary electrons in the beam. If the above conditions are satisfied, substantially complete beam current will ow and will be collected at the collector.

The manner for establishing the required potential distribution between pole piece 18 and collector 19 now will be explained by referring to FIG. 2 and by using the potentials assigned to the respective components inthe description of FIG. l. The potential V1 of pole piece 13 (first plane) is ground potential, and the depressedV potential V2 of collector 19 (second plane) is -6500 volts. However, using cathode potential as a reference, we may write V, 3500 vous p35 Tf1-10,000 volts Therefore, We tirst go up on ordinate axis o of FIG. 2 to the position .35. Next we move parallel to the abscissa axis o, through the minimum of the curves until we intercept a curve along its positive-slope portion, in this example the curve designated A. If the potential minimum of this curve is sutliciently below the potential V2 at the second plane (the un-normalized potential at the intercept of curve A) to repel the secondary electrons, yand is above cathode potential, the potential distribution represented by curve A will satisfy `our requirements. The ordinate of the potential minimum-of this curve A is .2, which in our example represents a potential of 2000 volts above cathode. Since the potential V2 of the collector is 3500 volts above cathode, the potential minimum of curve A will satisfy our requirements that the minimum be at least 30 to 40 volts below the collector voltage.

Having determined that we have chosen a satisfactory curve, we next determine the fr coordinate of the intercept chosen on curve A, which in FIG. 2 is approximately 1.96. As previously defined V1 is 10,000 volts above cathode, and the value of .l already is determined, as explained earlier. Therefore, the value of x0 is known and the separation between the pole piece 18 and collector 19 is equal to 1.96x0. This separation corresponds to the .theoretical separation x expressed in Equation l above. With this separation between pole piece 18 land collector 19, and if spaced parallel plane-space charge conditions are assimilated, the potential distribution between said pole piece and collector will be substantially as illustrated by the selected portion of curve A of FIG. 2, and our desired potential minimum will be established at a position corresponding to that expressed by Equation 2 above.

The thin apertured disc 20 at cathode potential aids in assimilating the selected theoretical condition having our desired potential distribution. The yaxial position and the :aperture diameter of apertured disc 20 which will establish the required potential distribution are best determined with the aid of an electrolytic tank. In the electrolytic tank, two potential planes representing V1 and V2 are established with the separation x as determined above. it a flat tank is used, the tank is inclined to establish a waterline extending perpendicularly between the two potential planes, this waterline corresponds to the beam axis. A strip of insulating material is placed parallel to the waterline and spaced therefrom by a distance representing the beam radius. A thin iiat conductive plate at a potential corresponding -to the cathode potential is then introduced edgewise into the electrolyte with its planar surfaces perpendicular to the waterline. This thin conductive plate cor-responds to apertured disc 20 of FIG. 1. The thin dat plate is empirically positioned axially and transversely in the tank until probing with a detector determines that the desired potential distribution along the beam edge, the dielectric strip in the tank, is established. The transverse separation of the edge lof the thin ilat plate from the waterline establishes the aperture radius of the disc 20, and its axial position between the two potential planes establishes the axial position of apertured disc 20 between pole piece 18 and collector 19. A more detailed discussion of the use of an electrolytic tank for eld mapping Iappears on pages 76-82 of Vacuum Tubes, by Spangenberg, published by McGraw-Hill, New York, NY., 1948.

Having determined all the necessary parameters to establish the required potential distribution for suppressing the secondary electrons, a tube incorporating these parameters now may be constructed which will operate in accordance with the present invention.

FIGS. 7 and 8 are curves which have been calculated on the basis of a given set of beam characteristics and a given sized thin apertured disc which forms the electrostatic field forming means, and from these curves the axial position and aperture diameter of the apertured disc may be determined without resorting to the electrolytic tank field mapping described above. The abscissa axes of FIGS. 7 and 8 are in terms of the normalized axial distance of the thin apertured disc from the first potential plane, pole piece 18, and the ordinate axes 1.0, and whose normalized diameter was Auxo. FIG.

8 is based on similar conditions except that normalized axial extent of the thin apertured disc was assumed to be .lo'xo instead of .Za'xo as in FIG. 7. To obtain the desired information from the curves of FIGS. 7 and 8, the curves of FIG. 2 first are used in the manner previously described to select the normalized spacing between the two planes which will give the desired potential minimum qa min. for the potentials on the respective planes. Obtaining from FIG. 2 the values of o min. and cr, these values are used to fix a point on the curves of FIGS. 7 ,and 8, wherein the longer curves running generally upwardly are curves of constant a and the shorter curves running transversely are curves of constant gb min. Taking the values previously obtained from FIG. 2 in the above description, i.e., qb min.=.2 and r=l.96, and locating a point on FIG. 7 fixed by these values, point x, we may now determine the `approximate normalized aperture dimension and axial position of the thin apertured disc. The abscissa coordinate (the normalized distance from the first potential plane) is approximately .73, .and the ordinate axis (the normalized distance from the beam edge) is approximately .32. With these values the axial position and aperture diameter of the thin apertured disc now may be established.

Several advantages result from electrically biasing field forming means at cathode potential. First, the field forming means consumes negligible power, and secondly, it helps to drain out the positive ions which otherwise might have collected in the region adjacent the collector. Because the accumulation of positive ions would tend to nullify the potential minimum, it is very desirable that they not be allowed to accumulate.

In order that the electrostatic field forming means comprised of apertured disc 29 be able to establish the desired potential distribution it must be very thin and should appear electrically as a line source of potential. Prior art electrodes having `appreciable axial extent cannot establish the desired axial potential distribution, and therefore must be distinguished from the structure employed in the present invention. In accordance with the present invention, an apertured disc having an axial thickness no greater than two-tenths of the axial separation between the two potential planes will perform its intended function.

A klystron amplifier tube constructed according to the present invention is illustrated in FIG. 3 wherein the left end of the tube is comprised of an electron gun having an electron-emissive surface 3i) and a heater element 31 which is electrically connected to a source of potential Vh. A focusing electrode 32 of the type disclosed in U.S. Patent 2,564,743 is disposed in front of and coaxial with emissive surface 30. The klystron tube further is comprised of entrance and exit reentrant cavities 33 and 34 which respectively are coupled to input waveguide section 35 and output waveguide section 36. The side walls of cavities 33 and 34 are formed by the internal cylindrical surface of member 37 which is a block of non-magnetic conductive material such as copper. The ends 38 and 39 of a non-magnetic conductive member 40 comprise the reentrant portions of cavities 33 and 34. Member 40 has a central aperture therethrough which serves as a drift tube between cavities 33 and 34. Magnetic pole pieces 41 and 42 are disposed at opposite ends of cavities 33 and 34 and are centrally apertured to permit the passage of the electron beam established by said electron gun. Pole pieces 4]. and 42 are externally connected to a source of magnetic ux (not shown) for establishing an axially directed steady magnetic field to focus the electron beam which passes through the electron beam-microwave energy interaction structure comprised of cavities 33 and 34. Collector electrode 43 is disposed at the right end of the tube for terminating said electron beam. The collector electrode includes at its right end a section of copper tube 44 which is pinched-off to provide a vacuum tight seal. Prior to the closure of said copper tube 44, evacuating means are connected thereto to evacuate said tube. A cylindrical cup 45 is disposed about collector 43 and serves as the outer housing for said collector.

Disposed intermediate pole piece 42 and collector 43, and electrically insulated therefrom, is an electrostatic field forming means comprised of an apertured disc 46 which is secured within a hollow cylinder of insulating material 47.

Pole pieces 41, 42 and the electron beam-microwave interaction structure comprised of cavities 33 and 34 are electrically insulated from the electron emitting structure of the tube by means of a glass-to-metal seal 48, and are electrically insulated from the collector section of the tube by a similar glass-to-metal seal 49 and by insulating tube 47. In accordance with the present invention, pole pieces 41, 42 and the interaction structure comprised of cavities 33 and 34 are operated at a positive potential with respect to electron emitting means 30, and collector 43 is operated at a depressed potential below the potential of said interaction structure. Further in accordance with the present invention, apertured disc 46 preferably is operated at the same potential as electron emitter 30.

The operation of the klystron tube illustrated in FIG. 3 as a microwave amplifier is conventional in that microwave energy is coupled into cavity 33 from input waveguide 35 and gives up energy to the electron beam passing through cavity 33 and causes velocity modulation of electrons in said beam. Said electron beam is caused to be density modulated in passing through the drift spaces between cavities, and said modulated beam gives up energy in cavity 34, this energy being extracted by means of output waveguide section 36.

The arrangement for suppressing the secondary electrons which are created at depressed collector 43 is substantially identical to the arrangement illustrated in FIG. 1. In the klystron tube of FIG. 3, the region at which the electron beam emerges from pole piece 42 constitutes a first potential plane at a potential V1 and the region at which said electron beam enters collector 43 constitutes the second plane at a potential V2. With the aid of the curves of FIG. 2 an axial separation x between said members is selected which will produce the desired type of potential distribution along the beam boundary. Then corresponding conditions are set up in an electrolytic tank and an electrode at electron emitter potential is introduced into the tank and empirically positioned radially from the beam boundary and axially between the two potential planes until the axial potential distribution along the beam boundary assimilates that of the curve chosen from FIG. 2. This electrode corresponds to the thin apertured disc 46, and thus determines the aperture diameter and axial position of said apertured disc.

The traveling wave tube illustrated in FIG. l and the klystron tube illustrated in FIG. 3 both employ magnetic pole pieces adjacent the input and output regions of the interaction structures as means to aid in the focussing of the electron beam. This is a common construction for these types of tubes, bu-t -it will be appreciated by those skilled in the art that the pole pieces may be positioned differently with respect to the input and output regions of the tube, and in some types of tubes internal magnetic pole pies are not employed at all. In the discussion of FIGS. l and 3 the position of the first potential plane was established at the end of the respective pole pieces 18 and 50 in accordance with the particular construction illustrated in said figures. But it is to be understood that regardless of the exact position of the pole piece relative to the output region of ythe tube, or if internal pole pieces are not used, the first potential plane will be located at the struct-ure closest to :the collector which is at or very near to the potential of the interaction region, or beam utilization region., of the tube. If a pole piece is located in this region, it is electricaily biased at or near the potential of the interaction structure of lthe tube (this potential being the thing of interest in determining the plane at potential V1), so in the attached claims, yfor convenience in describing the position of the first potential plane it will be considered that said pole piece in this instance comprises part of the interaction structure of the tube.

The embodiments of the invention illustrated in FIGS. l and 3 which employ a single thin apertured disc 20 as the electrostatic field forming means is quite satisfactory for D C. operation with small signal modulation and for pulsed operations with depressed co-llector potentials slightly in excess of 65%. Operation under these conditions virtually assures that during the operation of the tube the required potential distribution along the beam boundary will be stable so that the desired potential minimum will be sufficiently above cathode potential to permit primary electrons to flow to the collector and to block the secondary electrons and return them to the collector. This is the type C potential distribution referred to inthe above-referred article.

iFor large signal modulation and for even greater collector potential depression, it is advantageous to employ more =than one thin apertured disc for the eld forming means inasmuch as it has been found that in the presence of these latter-mentioned conditions the potential distribution may become unstable and may cause the formation of a virtual cathode (type B potential distribution referred to in the above-cited article), or at the opposite extreme, the potential minimum may disappear enirely. An arrangement of this latter type is illustrated in FIG. 4 which is a view of the collector region of a linear electron beam tube showing pole piece 50 and collector 51 axially spaced and electrically insulated from each other by dielectric insulating cylinder 52. Disposed in the region between pole piece 50 and collector 51 are three thin apertured discs S3, 54 and 55 which comprise electrostatic field forming means for establishing the beam boundary condition of the type under consideration, wherein the axial potential distribution between pole piece 50 and collector 51 has the desired potential minimum for repelling the secondary electrons back to collector 51. We have found that two or more thin apertured discs may be used. The axial and radial positions of the apertured discs, and the potentials applied thereto may vary considerably in this arrangement. At least one of the discs should be at cathode potential. Again, the positions of the discs and the necessary potentials applied thereto are best determined experimentally with the aid of an electrolytic tank, once the appropriate axial separation is chosen between the two potential planes.

Although the above discussion has thus far dealt in detail with the case wherein the condition established between the microwave-electron beam interaction structure and the collector assimilates that of a space charge condition between infinitely extending spaced parallel planes at different potentials (parallel electron beam flow), the present invention is equally applicable to suppressing secondary electrons in a depressed collector environment where the respective electrodes are, or assimilate, concentric cylindrical surfaces or concentric spherical surfaces, and wherein the electron beam is converging or diverging. FlG. 5 is a diagrammatic sectional view of the collector region of an electron beam tube wherein the opposing surface of pole piece 61 and collector 62 are surfaces which approximate spherical surfaces, and where the electron beam is diverging in the region between pole piece 61 and collector 62. Inaccordance with the invention, electrostatic eld forming means comprised of thin apertured disc 63 is positioned in the region between pole piece 61 and collector 62. An insulating cylindrical member 64 supports apertured disc 63 in its required position, as determined in the manner described above, and electrically insulates apertured disc 63 from pole piece 61 and collector 62. Further in accordance with the present invention, apertured disc 63 is electrically biased at cathode potential, and collector 62 is electrically biased at a depressed potential lower than the potential of the microwave interaction structure, of which pole piece 61 is considered a part.

An alternative embodiment of a similar type is illustrated in FlG. 6 where the electron beam is converging in ,the region between the pole piece 61 and collector 62. In View of the converging beam illustrated therein, the opposing surfaces of pole piece 61 and collector 62 approximate spherical surfaces having inverse curvatures from the cnrvatures illustrated in FIG. 5, otherwise, the construction and operation is similar to that illustrated in FIG. 5.

An article by I. Itzkan appearing in the April 1960 issue of Journal of Applied Physics, page 652, presents potential distribution curves of the type C distribution for the case of concentric spherical electrodes, and from an article by Page and Adams in The Physical Review, vol. 76 (1949), page 381, information is presented for obtaining similar type C curves for the case of concentric cylindrical electrodes. In determining the spacing for the two electrodes (pole piece 61 and collector 62), for these latter two cases, curves obtained from the above-cited articles are used instead of the curves of FIG. 2, which apply to the parallel plane situation. Otherwise, the procedure for determining the axial position and aperture diameter of apertured disc 63 in FIGS. 5 and 6 is the same as previously described for the arrangement illustrated in FIGS. l and 3. y While the invention has been described in its preferred embodiments, it is to be understood that the words which have been used Iare words of description rather than limitation and that changes within the purview of the appended claims may be made without departing from the true scope and spirit of the invention in its broader aspects.

What is claimed is:

1. In a linear electron beam device operating with a depressed collector potential the combination comprising electron emitting and beam forming means for producing an axially extending electron beam having known perve ance and radius, the radius of the beam being small compared to the axial extent of the beam, an electron beam collector axially spaced from said emitting means for terminating said beam, said collector having a surface 6X- tending radially from a point on sa-id axis and being electrically biased at a positive potential with respect to said emitting means, electron beam utilization means disposed intermed-iate said emitting means and said collector, an axially apertured member positioned adjacent to the end of said utilization means nearest said collector, said member being electrically biased at a potential more positive than the potential of said collector, and electrostatic field forming means disposed coaxially about and outside of said beam in the region between said member and said collector for presenting substantially a line source of p0- tential about said beam, said electrostatic ield forming means comprising an apertured, gridless disc having an axial thickness no greater than two-tenths of the axial separation between said member and said collector, the apertured, gridless disc be-ing electrically biased at the potential of the emitting means, the axial separation of said member and said collector, the axial position of said electrostatic eld forming means between said member and said collector, the radial spacing of the inner edge of the aperture of said gridless disc from the electron beam, and the characteristics of the electron beam being proportioned to assimilate a space charge condition between said member and collector wherein there is established along said beam boundary a predetermined axial potential distribution that decreases in going away from said member and reaches a minimum intermediate said member and collector and wherein the transverse potential gradient through said beam is substantially zero at said minimum, said minimum potential being greater than the potential of said emitter means to permit substantially complete electron beam current tlow to said collector, said axial potential distribution then rising at least 30 volts from said minimum to reach the potential at the collector.

2. The combination claimed in claim 1 wherein said collector 'has a spherically shaped surface that is disposed symmetrically about the axis of said electron beam.

3. The combination as claimed in claim 1 wherein said eld forming means is comprised of a plurality of axially spaced thin apertured discs disposed symmetrically around said beam, at least one of said apertured discs being electrically biased at the potential of the electron emitting means.

4. The combination as claimed in claim 1 wherein the configuration oi said electron beam in the region between said utilization means and said collector is that of a paral-y lel ow beam.

5. The combination as claimed in claim 1 wherein the configuration of said electron beam in the region between said utilization means and said collector is that of a convergent beam.

6. The combination as claimed in claim 1 wherein the configuration of said electron beam in the region between said utilization means and said collector is that of a divergent beam.

7. The combination claimed in claim 1 wherein said device is a traveling wave tube and wherein said beam utilization means is comprised of a slow wave propagating structure for permitting1 interaction of electromagnetic waves with said beam.

8. The combination claimed in claim 1 wherein said device is a klystron tube and said beam utilization means is comprised of at least one cavity resonator having electron permeable apertures therein for the passage of Said beam therethrough.

9. In a linear electron beam device operating with a depressed collector potential, the combination comprising an electron emitter for producing an axial beam of electrons having a radius r and a preveance K, an electron beam collector having a radially extending surface and being axially spaced from said emitter for terminating said beam, said collector being at a positive potential with respect to said emitter, electron beam utilization means disposed intermediate said emitter and said collector, an axially apertured member positioned adjacent the end of said utilization means nearest said collector, said member having a radially extending surface facing said collector and being electrically biased at a potent-iai more positive than the potential of said collector, and electrostatic iicld forming means comprising at least one planar, gridless, axially-thin apertured disc disposed coaxially about said beam in the region between said member and said collector and electrically biased at the potential of the emitter for producing along the beam boundary between said member and said collector an axial potential distribution that has a negative slope in the region adjacent said member, a positive slope in the region adjacent said collector and a potential minimum above emitter potential at a distance substantially equal to from said member, said potential minimum having a zero gradient in a transverse direction through said beam and the magnitude of said potential minimum being at least 30 volts below the potential of said collector, said collector and said member being axially separated by a distance x substantially equal to X=t +2rod-aowftwzaod-aotnxu wherein p is the ratio of the potential of the collector to the potential of said member, a is the ratio of the potential minimum to the potential of said member, and

10. An electron beam microwave energy interaction device comprising an electron emitter for establishing an electron beam of a given perveance K and radius r along an axis, an electron collector disposed along said axis in spaced relationship from said emitter, said collector having a radially extending surface and being electrically biased at a positive potential with respect to said emitter, an electron beam utilization region disposed along said axis between said emitter and said collector, a radially extending apertured surface at the collector end of said utilization region through which said beam exists from said utilization region, said surface being electrically biased at a positive potential with respect to said collector, said surface and said collector assimilating two spaced, parallel potential planes transverse to said beam, and electrostatic eld forming means at electron emitter potential presenting substantially a line source of potential positioned between said two planes, said eld forming means comprising an axially-thin, gridless, apertured disc extending transversely to said electron beam, the apertured surface at the end of the utilization region and the collector being separated by a distance x substantially equal to said iield forming means being axially positioned between the apertured surface of the utilization means and the collector and its inner surface being radially spaced from the beam boundary by distances to produce in the axial potential distribution between said planes a voltage minimum at an axial distance from the apertured surface, said Voltage minimum having a zero potential gradient in a transverse direction through said beam and having a magnitude above the potential of said emitter and at least 30 volts below the collector potential, said quantities p, a and x0 representing respectively the ratio of the potential at the collector to the potential of the apertured surface of the utilization means, the ratio of the potential at the potential minimum to the potential of said apertured surface, and

References Cited in the iile of this patent UNITED STATES PATENTS 2,240,183 Hahn Apr. 29, 1941 2,284,733 Haef June 2, 1942 2,508,316 Verburg et al. May 16, 1950 2,515,997 Haei July 18, 1950 2,949,558 Kompfner et al Aug. 16, 1960 2,956,199 Briskin Oct. 11, 196C 2,957,983 George Get. 25, 1960 2,991,391 Beaver July 4, 1961 

1. IN A LINEAR ELECTRON BEAM DEVICE OPERATING WITH A DEPRESSED COLLECTOR POTENTIAL THE COMBINATION COMPRISING ELECTRON EMITTING AND BEAM FORMING MEANS FOR PRODUCING AN AXIALLY EXTENDING ELECTRON BEAM HAVING KNOWN PERVEANCE AND RADIUS, THE RADIUS OF THE BEAM BEING SMALL COMPARED TO THE AXIAL EXTENT OF THE BEAM, AN ELECTRON BEAM COLLECTOR AXIALLY SPACED FROM SAID EMITTING MEANS FOR TERMINATING SAID BEAM, SAID COLLECTOR HAVING A SURFACE EXTENDING RADIALLY FROM A POINT ON SAID AXIS AND BEING ELECTRICALLY BIASED AT A POSITIVE POTENTIAL WITH RESPECT TO SAID EMITTING MEANS, ELECTRON BEAM UTILIZATION MEANS DISPOSED INTERMEDIATE SAID EMITTING MEANS AND SAID COLLECTOR, AN AXIALLY APERTURED MEMBER POSITIONED ADJACENT TO THE END OF SAID UTILIZATION MEANS NEAREST SAID COLLECTOR, SAID MEMBER BEING ELECTRICALLY BIASED AT A POTENTIAL MORE POSITIVE THAN THE POTENTIAL OF SAID COLLECTOR, AND ELECTROSTATIC FIELD FORMING MEANS DISPOSED COAXIALLY ABOUT AND OUTSIDE OF SAID BEAM IN THE REGION BETWEEN SAID MEMBER AND SAID COLLECTOR FOR PRESENTING SUBSTANTIALLY A LINE SOURCE OF POTENTIAL ABOUT SAID BEAM, SAID ELECTROSTATIC FIELD FORMING MEANS COMPRISING AN APERTURED, GRIDLESS DISC HAVING AN AXIAL THICKNESS NO GREATER THAN TWO-TENTHS OF THE AXIAL SEPARATION BETWEEN SAID MEMBER AND SAID COLLECTOR, THE APERTURED, GRIDLESS DISC BEING ELECTRICALLY BIASED AT THE POTENTIAL OF THE EMITTING MEANS, THE AXIAL SEPARATION OF SAID MEMBER AND SAID COLLECTOR, THE AXIAL POSITION OF SAID ELECTROSTATIC FIELD FORMING MEANS BETWEEN SAID MEMBER AND SAID COLLECTOR, THE RADIAL SPACING OF THE INNER EDGE OF THE APERTURE OF SAID GRIDLESS DISC FROM THE ELECTRON BEAM, AND THE CHARACTERISTICS OF THE ELECTRON BEAM BEING PROPORTIONED TO ASSIMILATE A SPACE CHARGE CONDITION BETWEEN SAID MEMBER AND COLLECTOR WHEREIN THERE IS EATABLISHED ALONG SAID BEAM BOUNDARY A PREDETERMINED AXIAL POTENTIAL DISTRIBUTION THAT DECREASES IN GOING AWAY FROM SAID MEMBER AND REACHES A MINIMUM INTERMEDIATE SAID MEMBER AND COLLECTOR AND WHEREIN THE TRANSVERSE POTENTIAL GRADIENT THROUGH SAID BEAM IS SUBSTANTIALLY ZERO AT SAID MINIMUM, SAID MINIMUM POTENIAL BEING GREATER THAN THE POTENTIAL OF SAID EMITTER MEANS TO PERMIT SUBSTANTIALLY COMPLETE ELECTRON BEAM CURRENT FLOW TO SAID COLLECTOR, SAID AXIAL POTENTIAL DISTRIBUTION THEN RISING AT LEAST 30 VOLTS FROM SAID MINIMUM TO REACH THE POTENTIAL AT THE COLLECTOR. 